Die materials used extensively by the glass industry include metallic mold materials, such as cast iron, stainless steel (e.g., AISI 431 stainless steel), cast bronze, metallic alloys (e.g., Ni-based alloys), and precious metals (e.g., Pt). These glass molding dies and forming tools operate in air and at elevated temperatures (up to 1000° C.), while the working surface is exposed to the chemically active molten glass and subjected to thermomechanical cyclic operations. These severe process conditions result in three critical problems with respect to the performance and reliability of glass molding dies: oxidation, sticking/adhesion by molten glass, and wear.
Furthermore, modem glass-making process requirements have placed a greater demand on the performance of materials used for glass molding dies and forming tools. For instance, glass quality requirements are greater, process temperatures are higher, closer control of dimensional tolerances is desired, longer service life is expected, and higher productivity has become an economic necessity. All of these requirements have pushed the demands on the properties and performance of die materials to higher and higher levels.
In recent years, efforts have been directed to enhance the performance of glass molding dies and forming tools, and to improve the quality of glass products. One method to improve the performance of glass molding dies is optimization of chemical composition and structure of die materials themselves. Another effort has been made to identify new potential mold materials. For instance, the replacement of cast iron by steels, non-ferrous alloys, sintered ceramics, and composites. However, the replacement of conventional die materials does not always ensure success because there are accompanying changes in the physical, mechanical, chemical and operational characteristics responsible for determining tool reliability and life.
The problem associated with the metallic die materials such as stainless steel and various refractory alloys, is their poor resistance to oxidation at temperatures exceeding 600° C. Glass molding using a die made of such metallic materials must be conducted in an atmosphere protected with nitrogen gas. Therefore, the forming tools made of these metallic materials have reduced durability and lower productivity.
To improve the press-molding dies, a die composed of cemented carbides (WC) coated with a thin layer of nickel-precious metal alloy was proposed in a Japanese Unexamined Patent Publication No. SHO 64-61327. Although the proposed die is superior in mechanical strength and easy to process, it still has the disadvantage that the highest working temperature is 600° C. because of its poor oxidation resistance and adhesion/sticking to molten glass.
Other ceramic dies include, for example, a die of mixed materials including titanium carbide (TiC) and a metal, as disclosed in Japanese Unexamined Patent Publication No. SHO 59-121126. Although a die made of TiC and a metal is hard and mechanically strong, it is very difficult to machine the dies precisely. In addition, the die has the problem of reacting with components of glass, such as lead or alkaline metals.
As disclosed in U.S. Pat. No. 5,306,339, a ceramic die for glass molding characterized by having the press surfaces formed of boron type composite ceramics comprising (1) at least one MB ceramic phase (wherein M is Ni, Cr, V, Nb, Ta, Mo, W or Mn) having a M/B atomic ratio of 1/1 and (2) at least one Group IV diboride ceramic phase selected from TiB2, ZrB2, or HfB2, and/or a (Cr,Ni)3B4 ceramic phase. This glass molding die was reported to produce enhanced glass productivity in the open air at elevated temperatures up to 750° C. Nevertheless, if the highest process temperature is required to be greater than 750° C. for some glasses, this type of die made of diborides will be deficient in resistance to oxidation.
Another method of increasing the life and performance of glass molding dies and tools includes surface modifications and coatings that are gaining increasingly widespread industrial acceptance as one of the most important and versatile means of improving component performance, particularly as a means of providing improvements in tool reliability, service life and product quality.
U.S. Pat. No. 5,405,652, to Kashiwagi et al. discloses a method of manufacturing a die for use in molding glass optical elements by sputter coating of platinum and iridium noble metal alloys. These dies were compared with those made of SiC sintered material, and it was demonstrated that the noble metal die coatings were superior with respect to glass adhesion problems, however, the price of the die was increased dramatically. On the other hand, the test was carried out only in a nitrogen atmosphere and the highest glass temperature was only 500° C. In practice, there are a large number of glass forming processes requiring higher temperatures and operating in air.
For production of a glass substrate for a magnetic disk that is suitable as a recording medium, a press-molding die, according to U.S. Pat. No. 6,119,485, comprises a base material, an intermediate layer formed on the surface of the base material and a protective layer formed on the intermediate layer. The base material has an inorganic oxide such as silicon oxide or aluminum oxide. The intermediate layer includes a material which adheres to both the base material and the protective layer and the protective layer has at least one metal film consisting of tungsten (W), platinum (Pt), palladium (Pd), ruthenium (Ru), iridium (Ir), Osmium (Os), rhenium (Re), tantalum (Ta) or alloys of these materials.
A similar press-molding die is disclosed in U.S. Pat. No. 5,538,528. The invention provides a die for press-molding a glass optical element having a base material and a tantalum-containing alloy thin layer on the surface. The sintered base material has at least one material such as tungsten carbide (WC), titanium carbide (TiC), or titanium nitride (TiN). The 1 to 5 μm tantalum (Ta)-containing alloy thin layer contains Ta and at least one other element such as platinum (Pt), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium (Os), rhenium (Re), tungsten (W), and palladium (Pd). These two inventions provide a press-molding die with high precision and little deterioration in manufacturing magnetic disk substrates and optical glass elements. There is no disclosure of the limitations of these inventions as far as process temperature and adhesion between glass and die are concerned. However, it is apparent that it is not an economically sound approach to use these precious and rare earth elements selected in the inventions, especially for a large glass molding die or forming tools.
Some coating materials have been studied for glass molding dies, e.g., plasma nitrided coatings, galvanic plated chromium coating, paint-on ceramic coatings, such as Si3N4, SiC and BN, and PVD-produced AlN, TiN, TiAlN coatings. Although TiAlN coated dies have clearly shown lower wear and deterioration than uncoated tool surfaces, sticking of molten glass to the TiAlN coated dies was not decreased. In comparison with the other materials, BN-coated mold specimens showed desirable non-sticking behavior by the glass melt in the temperature range between about 500° C. and about 750° C. Similar results have been obtained by the synthesis of BN thin films by ion beam and vapor deposition (IVD) processes. The films were multilayered BN films, in which B-rich BN films were formed on the substrate, and stoichiometric films were deposited on the B-rich BN films. This showed excellent tribological properties of the multilayered BN coated WC molding die for forming borosilicate glass lenses at 630° C. Measurements of contact angles of Duran glass (corresponding to the International Standard “Borosilicate glass 3.3” according to ISO/DIN 3585) on hexagonal boron nitride both in air and in vacuum have been conducted by the present inventors. Boron nitride showed promising non-sticking behavior by glass in vacuum. However, during experiments conducted in air, the formation of bubbles in the molten glass were observed at 950° C., and grew with increasing temperature. The bubbles broke at a certain size, and then new bubbles grew. It is believed that the formation of bubbles is due to the oxidation of BN and the release of B2O3 gas. As a result, the wetting of boron nitride by glass accelerated. Thus, boron nitride is not a suitable coating material for glass molding dies to operate at a process temperature higher than 900° C.
The progress made in recent years to improve the performance of glass molding dies has largely been concerned with solving only one or two aspects of the problems associated with glass molding dies. None of the solutions proffered thus far has fulfilled all the characteristics that the glass molding dies and forming tools are required to possess. Thus, there exists a need for glass molding dies and forming tools that do not stick to the molten glass, possess high oxidation resistance, high wear resistance, low coefficients of friction and high thermal shock resistance (thermal fatigue resistance).